Error B7 257

Safety precautions:

- If you smell gas, leave the property immediately and call the gas emergency number. Do not operate electrical switches or the boiler. Call a Gas Safe engineer.

- Before any inspection switch off the boiler at the main switch and isolate electrical supply at the consumer unit if you will be opening the casing. Allow the boiler to cool.

- Do not attempt to work on gas valves, the burner, or any live high-voltage parts unless you are a qualified Gas Safe engineer.

Initial homeowner checks (safe and simple):

1. Note the code exactly (B7 257) and any other codes or flashing lights. Photograph the display for the engineer.

2. Try a simple reset: press and hold the boiler reset button as described in your manual (usually 3 seconds). Wait and see if the boiler clears and attempts to fire. If it resets once but returns to B7 257, further diagnosis is needed.

3. Check mains power and fuses: ensure the boiler has power (display illuminated). Check the domestic fuse/RCBO and any inline fuses to the boiler. If the boiler is dead as well as showing the code, mention this to the engineer.

4. Check external controls: ensure room thermostat, programmer/timers, and any wired controls are calling for heat (sometimes an external control fault changes behavior) — but the B7 257 is an internal control fault, so external controls rarely fix it.

Safe diagnostic steps you may perform if comfortable and only after isolating power:

1. Isolate power to the boiler at the fused spur/consumer unit. Confirm the unit is off and the display is dark.

2. Remove the boiler casing following the manufacturer's instructions (if you are confident and the manual permits). Do not access gas or burner components.

3. Visually inspect the control area and PCBs for obvious signs of damage: burn marks, bulging capacitors, corrosion, water or moisture, loose connectors, or a dislodged KIM/coding card.

4. If you see a removable coding card/KIM that looks loose and you are confident to proceed, power remain isolated and gently reseat the module into its socket then refit casing. Only reseat connectors that are obviously loose; do not force or bend pins.

5. Re-energise the boiler and attempt a reset. If the fault clears and the boiler runs normally, monitor for reoccurrence and book a service — intermittent electronic faults often precede permanent failure.

6. If the fault persists, or you found obvious PCB damage, water ingress, or burnt components, switch the boiler off and call a Gas Safe engineer. Do not attempt further electronic repairs yourself.

What a professional will do (so you know what to expect):

- A Gas Safe engineer will run the boiler diagnostics and read cause codes from the information menu, measure supply voltages, test wiring and connectors, and check communication between the HCM/KIM and the burner control unit.

- They will carry out safe electrical tests, may replace or reseat the KIM/coding card, and if necessary replace the burner control unit / PCB with manufacturer-approved parts. After repair they will test the burner, combustion, and safety devices and clear the fault memory.

When to call a professional now:

- If reset does not clear the code, if the code returns, if you see physical damage, water ingress, burnt smell, or if you are unsure about opening the casing — call a Gas Safe registered engineer. This error affects burner control and must be resolved correctly for safe operation.

Additional practical advice:

- Have the boiler model, serial number, and a photo of the display and any visible fault codes ready for the engineer. Note when the boiler was last serviced.

- Arrange a service if you have not had one in the last 12 months; regular servicing reduces electronic and component failures.

Important: Do not attempt to bypass safety interlocks or jury-rig the boiler to run. Repairs involving control electronics, the gas supply, ignition system, or burner must be performed by a qualified Gas Safe engineer.